feat: (WIP) Let developers more easily override SQL column type to JS…

…ON schema mapping

singer_sdk/connectors/sql.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import functools
 import logging
 import typing as t
 import warnings
@@ -103,6 +104,61 @@ def prepare_part(self, part: str) -> str:  # noqa: PLR6301
         return part
 
 
+class SQLToJSONSchemaMap:
+    """SQLAlchemy to JSON Schema type mapping helper.
+
+    This class provides a mapping from SQLAlchemy types to JSON Schema types.
+    """
+
+    @functools.singledispatchmethod
+    def to_jsonschema(self, sa_type: sa.types.TypeEngine) -> dict:  # noqa: ARG002, PLR6301
+        """Return a JSON Schema representation of the provided type."""
+        return th.StringType.type_dict  # type: ignore[no-any-return]
+
+    @to_jsonschema.register
+    def datetime_to_jsonschema_datetime(self, sa_type: sa.types.DateTime) -> dict:  # noqa: ARG002, PLR6301
+        """Return a JSON Schema representation of a generic datetime type."""
+        return th.DateTimeType.type_dict  # type: ignore[no-any-return]
+
+    @to_jsonschema.register
+    def date_to_jsonschema_date(self, sa_type: sa.types.Date) -> dict:  # noqa: ARG002, PLR6301
+        """Return a JSON Schema representation of a date type."""
+        return th.DateType.type_dict  # type: ignore[no-any-return]
+
+    @to_jsonschema.register
+    def time_to_jsonschema_time(self, sa_type: sa.types.Time) -> dict:  # noqa: ARG002, PLR6301
+        """Return a JSON Schema representation of a time type."""
+        return th.TimeType.type_dict  # type: ignore[no-any-return]
+
+    @to_jsonschema.register
+    def integer_to_jsonschema_integer(self, sa_type: sa.types.Integer) -> dict:  # noqa: ARG002, PLR6301
+        """Return a JSON Schema representation of a an integer type."""
+        return th.IntegerType.type_dict  # type: ignore[no-any-return]
+
+    @to_jsonschema.register
+    def float_to_jsonschema_float(self, sa_type: sa.types.Numeric) -> dict:  # noqa: ARG002, PLR6301
+        """Return a JSON Schema representation of a generic number type."""
+        return th.NumberType.type_dict  # type: ignore[no-any-return]
+
+    @to_jsonschema.register
+    def string_to_jsonschema_string(self, sa_type: sa.types.String) -> dict:  # noqa: ARG002, PLR6301
+        """Return a JSON Schema representation of a generic string type."""
+        # TODO: Enable support for maxLength.
+        # if sa_type.length:
+        #     return StringType(max_length=sa_type.length).type_dict  # noqa: ERA001
+        return th.StringType.type_dict  # type: ignore[no-any-return]
+
+    @to_jsonschema.register
+    def boolean_to_jsonschema_boolean(self, sa_type: sa.types.Boolean) -> dict:  # noqa: ARG002, PLR6301
+        """Return a JSON Schema representation of a boolean type."""
+        return th.BooleanType.type_dict  # type: ignore[no-any-return]
+
+    @to_jsonschema.register
+    def variant_to_jsonschema_time(self, sa_type: sa.types.Variant) -> dict:  # noqa: ARG002, PLR6301
+        """Return a JSON Schema representation of a variant type."""
+        return th.TimeType.type_dict  # type: ignore[no-any-return]
+
+
 class SQLConnector:  # noqa: PLR0904
     """Base class for SQLAlchemy-based connectors.
 
@@ -156,6 +212,17 @@ def logger(self) -> logging.Logger:
         """
         return logging.getLogger("sqlconnector")
 
+    @functools.cached_property
+    def type_mapping(self) -> SQLToJSONSchemaMap:
+        """Return the type mapper object.
+
+        Override this method to provide a custom mapping for your SQL dialect.
+
+        Returns:
+            The type mapper object.
+        """
+        return SQLToJSONSchemaMap()
+
     @contextmanager
     def _connect(self) -> t.Iterator[sa.engine.Connection]:
         with self._engine.connect().execution_options(stream_results=True) as conn:
@@ -260,8 +327,8 @@ def get_sqlalchemy_url(self, config: dict[str, t.Any]) -> str:  # noqa: PLR6301
 
         return t.cast(str, config["sqlalchemy_url"])
 
-    @staticmethod
     def to_jsonschema_type(
+        self,
         sql_type: (
             str  # noqa: ANN401
             | sa.types.TypeEngine
@@ -287,7 +354,10 @@ def to_jsonschema_type(
         Returns:
             The JSON Schema representation of the provided type.
         """
-        if isinstance(sql_type, (str, sa.types.TypeEngine)):
+        if isinstance(sql_type, sa.types.TypeEngine):
+            return self.type_mapping.to_jsonschema(sql_type)
+
+        if isinstance(sql_type, str):
             return th.to_jsonschema_type(sql_type)
 
         if isinstance(sql_type, type):

tests/core/test_connector_sql.py

@@ -11,7 +11,7 @@
 
 from samples.sample_duckdb import DuckDBConnector
 from singer_sdk.connectors import SQLConnector
-from singer_sdk.connectors.sql import FullyQualifiedName
+from singer_sdk.connectors.sql import FullyQualifiedName, SQLToJSONSchemaMap
 from singer_sdk.exceptions import ConfigValidationError
 
 if t.TYPE_CHECKING:
@@ -392,3 +392,50 @@ def prepare_part(self, part: str) -> str:
 def test_fully_qualified_name_empty_error():
     with pytest.raises(ValueError, match="Could not generate fully qualified name"):
         FullyQualifiedName()
+
+
+@pytest.mark.parametrize(
+    "sql_type, expected_jsonschema_type",
+    [
+        pytest.param(sa.types.VARCHAR(), {"type": ["string"]}, id="varchar"),
+        pytest.param(
+            sa.types.VARCHAR(length=127),
+            {"type": ["string"], "maxLength": 127},
+            marks=pytest.mark.xfail,
+            id="varchar-length",
+        ),
+        pytest.param(sa.types.TEXT(), {"type": ["string"]}, id="text"),
+        pytest.param(sa.types.INTEGER(), {"type": ["integer"]}, id="integer"),
+        pytest.param(sa.types.BOOLEAN(), {"type": ["boolean"]}, id="boolean"),
+        pytest.param(sa.types.DECIMAL(), {"type": ["number"]}, id="decimal"),
+        pytest.param(sa.types.FLOAT(), {"type": ["number"]}, id="float"),
+        pytest.param(sa.types.REAL(), {"type": ["number"]}, id="real"),
+        pytest.param(sa.types.NUMERIC(), {"type": ["number"]}, id="numeric"),
+        pytest.param(
+            sa.types.DATE(),
+            {"type": ["string"], "format": "date"},
+            id="date",
+        ),
+        pytest.param(
+            sa.types.DATETIME(),
+            {"type": ["string"], "format": "date-time"},
+            id="datetime",
+        ),
+        pytest.param(
+            sa.types.TIMESTAMP(),
+            {"type": ["string"], "format": "date-time"},
+            id="timestamp",
+        ),
+        pytest.param(
+            sa.types.TIME(),
+            {"type": ["string"], "format": "time"},
+            id="time",
+        ),
+    ],
+)
+def test_sql_to_json_schema_map(
+    sql_type: sa.types.TypeEngine,
+    expected_jsonschema_type: dict,
+):
+    m = SQLToJSONSchemaMap()
+    assert m.to_jsonschema(sql_type) == expected_jsonschema_type